1. Field of the Invention
The present invention generally relates to methods and apparatuses for electric power conversion and, more particularly, to a method and an apparatus for electric power conversion operating between a dc power system and an ac power system optimized for efficient harmonic elimination.
2. Description of the Related Art
FIG. 20 shows a construction of a related-art electric power conversion apparatus shown in Generalized Techniques of Harmonic Elimination and Voltage Control in Thyristor Inverters: Part Ixe2x80x94Harmonic Elimination, IEEE Transactions On Industry Applications, Vol. IA-9, No. 3, May/June 1973. Referring to FIG. 20, the electric power conversion apparatus comprises a dc power system, a three-phase (u phase, v phase, w phase) ac power system 2, a power conversion unit 3 for dc to ac power conversion based on pulse width modulation for elimination of harmonics of specific orders and a codulating circuit 4 for controlling an output voltage of the power conversion unit 3. The power conversion unit 3 utilizes switching elements such as gate turn-off thyristors.
FIGS. 21A, 21B and 21C are graphs showing waveforms Vou, Vov and Vow of the u-phase voltage, v-phase voltage and w-phase voltage, respectively, output from the power conversion unit 3. Vsu, Vsv and Vs each represents a sinusoidal waveform of the fundamental of the output voltage for each phase. The waveforms Vsu, Vsv and Vsw are mathematically represented as
Vsu=kxc2x7Edcxc2x7sin xcex8xe2x80x83xe2x80x83(1)
Vsv=kxc2x7Edcxc2x7sin(xcex8xe2x88x92120xc2x0)xe2x80x83xe2x80x83(2)
Vsv=kxc2x7Edcxc2x7sin(xcex8+120xc2x0)xe2x80x83xe2x80x83(3)
where k indicates an instruction (control setting) for an amplitude of voltage of a fundamental, xcex8 indicates an instruction (control setting) for a phase of a voltage of a fundamental and Edc indicates a dc voltage level.
FIG. 22 is a numeral chart referred to in order to determine variables xcex11, xcex12, xcex13, xcex14 and xcex15 that define the sinusoidal waveforms Vsu, Vsv and Vsw. Instructions k input to the modulating circuit 4 are plotted horizontally in the chart. Each of the variables xcex11-xcex15 varies in a range of 0 to 90 degrees and is plotted vertically in the chart. For each given instruction k, a set of variables xcex11, xcex12, xcex13, xcex14 and xcex15 are determined.
FIG. 23 is a graph showing an amplitude of a third harmonic included in a waveform of a voltage for one phase produced by the electric power conversion apparatus.
A description will now be given of the operation according to the related art. Responsive to an input of a fundamental voltage amplitude instruction k and a fundamental voltage phase instruction xcex8, the modulating circuit 4 outputs an operation signal r for ensuring that the fundamental components of the voltages for the respective phases output from the power conversion unit 3 have respective waveforms Vsu, Vsv and Vsw. In other words, the modulating circuit outputs the operation signal r that causes the power conversion unit 3 to output the waveforms Vou, Vov and Vow.
In response to the operation signal r from the modulating circuit 4, the power conversion unit 3 effects power conversion between the dc power system 1 and the ac power system 2, by driving the switching elements so that the u-phase output voltage, the v-phase output voltage and the w-phase output voltage have the waveforms Vou, Vov and Vow, respectively.
As shown in FIGS. 21A, 21B and 21C, the voltage waveforms Vou, Vov and Vow are determined by the variables xcex11-xcex15. The variable xcex11-xcex15 are uniquely determined by the fundamental voltage amplitude instruction k. More specifically, the variables xcex11-xcex15 are determined by the modulating circuit 4 by referring to a numeral table of FIG. 22.
Each of the voltage waveforms Vou, Vov and Vow comprises five pulses symmetrical about the horizontal axis. The fundamental components of the voltage waveforms Vou, Vov and Vow have the waveforms Vsu, Vsv and Vsw. The amplitude of fifth, seventh, eleventh and thirteenth harmonics is zero.
With this operation, it is ensured that the fifth, seventh, eleventh and thirteenth harmonics are eliminated from the voltage output from the power conversion unit 3. The power conversion unit 3 outputs fundamental components having a predetermined amplitude. When the output voltages are in a three-phase balanced state, the line-to-line voltage cancels harmonics that are multiples of three so that these harmonics are not included in the output voltages.
As described above, the related-art power conversion apparatus is constructed such that harmonics that are multiples of three are cancelled by the line-to-line voltage and eliminated from the output voltages, when the output voltages of the power conversion unit 3 are in a three-phase balanced state. When imbalance of the output voltages of the power conversion unit 3 occurs, harmonics that are multiples of three are included in the output voltages.
More specifically, when imbalance of the output voltages of the power conversion unit 3 occurs (for example, when the output voltages of the respective phases have mutually different fundamental components so that the fundamental components have different phases), the magnitude of harmonics of multiples of three, included in the output voltage, differ from phase to phase. Consequently, harmonics of multiples of three cannot be canceled by the line-to-line voltage and are included in the output voltages.
In order to eliminate harmonics such as third harmonics or ninth harmonics, it is conceivable to use a plurality of power conversion units that employ the pulse width modulation method for eliminating harmonics of specific orders.
However, when a plurality of power conversion units are connected in parallel such that the phases of the fundamental components thereof are displaced from each other by an appropriate magnitude, using multiple reactors for connection, a cross current having a fundamental frequency may flow between the plurality of power conversion units. When a plurality of power conversion units are connected in series such that the phases of the fundamental components thereof are displaced from each other, the upper limit of the voltage of the fundamental that can be output is reduced.
Accordingly, an object of the present invention is to provide an electric power conversion apparatus in which the aforementioned problems are eliminated.
Another and more specific object of the present invention is to provide an electric power conversion apparatus capable of suppressing harmonics including third harmonics and ninth harmonics included in the output voltage even when phase-to-phase imbalance of the output voltages of a plurality of power conversion units, and without inducing a cross current having the frequency of a fundamental between the plurality of power conversion units.
The aforementioned objects can be achieved by a power conversion apparatus using a power conversion unit to effect power conversion between a dc power system and an ac power system, comprising: a plurality of power conversion units using a pulse width modulation method adapted for elimination of harmonics of specific orders and connected to the ac power system, wherein the plurality of power conversion units supply voltages of mutually different waveforms to the ac power system.
The plurality of power conversion units may be connected in parallel with the ac power system via reactors.
Two power conversion units may be connected in parallel.
Three power conversion units may be connected in parallel.
The aforementioned objects can also be achieved by a method of converting electric power between a dc power system and an ac power system, comprising the steps of: supplying instructions to a plurality of power conversion units each using a pulse width modulation method adapted for elimination of harmonics of specific orders and connected to the ac power system; supplying from the plurality of power conversion units voltages of mutually different waveforms to the ac power system.